Each cell in a wireless cellular network system is typically provided with a base station communicating with a user equipment. Before a data transmission process is started, the base station will transmit reference signals (i.e., pilot signals) to the user equipment, and the user equipment can obtain a value of channel estimation from the reference signals. The reference signals are a known sequence of signals transmitted at a specific prescribed time at a specific prescribed frequency. The quality of channel estimation will be influenced by interference, noise and other factors.
Typically the user equipment is located at different geographical locations with different received signal strengths and different noise and interference strengths. Thus some user equipments, e.g., a user equipment located at the center of a cell, can communicate at a higher rate, and some other user equipments, e.g., a user equipment located at the edge of a cell, can communicate only at a lower rate. Data is transmitted to the user equipment preferably in a format matching a channel condition of the user equipment in order to make full use of a transmission bandwidth of the user equipment. A technology to match a format in which data is transmitted to the user equipment with a channel condition of the user equipment is referred to as link adaptation.
The user equipment shall report a Channel Quality Indicator (CQI) according to a channel condition of the user equipment in order to assist the base station in performing link adaptation. The CQI reported from the user equipment corresponds to specific time and frequency resources, that is, the CQI reported from the user equipment represents a capability of transmission over these time and frequency resources. The CQI shall be calculated by the user equipment measuring interference and noise of an adjacent cell to the user equipment, including interference I and noise power N0.
The International Telecommunication Union (ITU) poses a very stringent requirement on the performance of a next-generation mobile communication system, for example, a largest transmission bandwidth of the system up to 100 MHz and a required peak rate of uplink and downlink data transmission up to 1 Gbps and 500 Mbps, and a very high demand for an average spectrum efficiency, particularly an edge spectrum efficiency, of the system. In order to satisfy the requirements on the new system, the 3GPP proposes in Long Term Evolution-Advanced (LTE-A) of a next-generation mobile cellular communication system the use of the technology of coordinated multi-point transmission to improve the performance of the system. The technology of coordinated multi-point transmission relates to coordination between a plurality of transmission points separated in geographical location. In general, the plurality of transmission points are base stations of different cells. The technology of coordinated multi-point transmission is divided into downlink coordinated transmission and uplink joint reception. The technology of downlink coordinated multi-point transmission is generally divided into two categories of joint scheduling and joint transmission. Joint scheduling refers to coordination of time, frequency and space resources between the cells to allocate resources orthogonal to each other for different User Equipments (UEs) in order to avoid mutual interference. Inter-cell interference is a major factor restricting the performance of a UE at the edge of a cell, and thus joint scheduling can lower inter-cell interference to thereby improve the performance of the UE at the edge of the cell. As illustrated in FIG. 1A, joint scheduling of three cells schedules three UEs with possible mutual interference onto resources orthogonal to each other to thereby avoid interference between the cells effectively.
Unlike a joint scheduling scheme in which data is transmitted from only one cell to a UE, data is transmitted concurrently from a plurality of cells to a UE in a joint transmission scheme to thereby enhance reception of a signal at the UE. As illustrated in FIG. 1B, data is transmitted from three cells to a UE over the same resource, and the UE receives signals of the plurality of cells concurrently. Superposition of the useful signals from the plurality of cells can improve the quality of the signal received at the UE on one hand and lower interference to the UE on the other hand to thereby improve the performance of the system.
The user equipment shall further estimate channel state information of a base station in a coordinating cell to the user equipment in addition to a serving cell in order to support coordinated multi-point transmission effectively. Channel state information is estimated by measuring a pilot in the LTE-A. Fig.1C illustrates a mapping relationship of pilots and data in a Physical Resource Block (PRB). The first two Orthogonal Frequency Division Multiplexing (OFDM) symbols are used for transmission of control information, and a data region starts with the third OFDM symbol. The data region includes pilot Resource Elements (REs) and data RE. A pilot of an adjacent cell will typically be mapped onto different REs. This is because a pilot RE is typically at high power and transmitted throughout a bandwidth and there will be very strong interference between pilots mapped onto the same RE to influence the precision of channel estimation. As can be apparent from Fig.1C, a user equipment in a cell 1 shall perform channel estimation on REs corresponding to measurement pilots of a cell 2 and a cell 3 to obtain channel state information of the cell 2 and the cell 3. On these REs, downlink data transmission (e.g., a Physical Downlink Shared Channel (PDSCH)) may be scheduled in the cell 1, that is, the pilots of the cell 2 and the cell 3 will suffer interference of the data transmission in the cell 1. For the user equipment in the cell 1, the strength of a signal received at the user equipment from the cell 1 will typically be far above those of signals of the cell 2 and the cell 3 so that the Signal to Interference and Noise Ratios (SINRs) of the measurement pilots of the cell 2 and the cell 3 will be very low and no channel estimation at satisfactory precision can be obtained. In order to address this problem, the cell 1 can mute those REs on which the cell 2 and the cell 3 transmit the measurement pilots, that is, the cell 1 transmits a signal at zero power at those REs, and this solution is referred to as RE MUTING, and the muted REs are referred to as MUTING REs (see FIG. 1D for details thereof).
The user equipment shall feed back a CQI by estimating interference to the user equipment itself, and interference of an adjacent cell is absent at the location of the pilots in the RE MUTING solution. Interference of an adjacent cell is typically the strongest, and thus the interference estimated at the locations of the pilots will be far below the real interference to the user equipment.
In summary the user equipment may measure interference at lower precision at present if the RE MUTING solution is adopted in the cell.